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    <title>Document</title>
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    /*动态类型声明变量不需声明数字类型 都用var*/
    var num=1;
    var str="hello";
    var b = true;
    /*js区分大小写
    变量命名不能以数字开头
    变量命名不能是关键字 保留字
    */
    var sum=10;
    var SUM=5;
    console.log(SUM);
    console.log(sum);
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